Among the sea of glossy black and white penguins that call a popular Australian aquarium home, one baby bird stands out like a sore thumb.

Chocolatey brown, obscenely fluffy, and towering a head above his own foster parents – plus weighing more than both combined – is Pesto.

Affectionately dubbed a “fatty”, an “absolute unit”, and a “linebacker”, the chonky chick has shot to viral superstardom and attracted a legion of obsessed fans, including popstar Katy Perry.

Pesto has reached an audience of billions – dominating social media algorithms, securing wall-to-wall breakfast TV coverage in the US and UK, and drawing travellers from around the globe to Melbourne’s Sea Life Aquarium.

Born in January weighing only 200g (7oz), the nine-month-old king penguin is now more than a hundred times the size. At a whopping 22.5kg (50lb), he is the biggest chick the aquarium has ever seen.

It is normal for penguins to stack on some “healthy baby chub” after they hatch, the aquarium’s Jacinta Early tells the BBC, but keepers had no idea Pesto would become so immense.

“It’s a combination of nature [and] nurture, really,” the marine biologist explains.

Pesto’s biological father is quite tall, but he’s also been very well looked after by his foster parents Tango and Hudson: “He eclipses them now, which also makes him look comically large.”

Hand-fed several times a day, Ms Early says Pesto’s considerable heft is also partly attributable to his “very healthy appetite” – a gentle way of saying he gobbles up to 30 fish daily.

But she stresses he’s healthy – essentially half fluff – and he’ll soon begin to drop weight naturally.

“If I poked him, my entire finger would completely [disappear] deep in his feathers,” Ms Early says.

“When he does start to fledge, he’ll lose a lot of that baby fluff, and he’ll also lose much of that weight, so he’ll slim down nice and sleek.”

He’s already losing a smattering of his baby feathers, but she says there’s much more to Pesto than his cuddly appearance anyway.

She describes him as a social butterfly who chatters away in a “cute little whistle tone” and loves annoying the adult penguins “like any typical toddler would”.

“He tends to be the first one to kind of say hi [to keepers] and he also does respond to his name.”

“We definitely do have our favourites,” Ms Early says, diplomatically. “[But] Pesto seems to be a little bit in love with all the keepers as well.”

Even though the keepers have long been besotted with the chick, seeing the love he’s attracted from the public has been an absurd experience, they say.

It’s a generally busy period of year for the aquarium, amid school holidays, but guests have been flocking to the penguin exhibit to catch a glimpse of the glorified pom-pom.

Millie Jacoby – a Brit who has lived in Melbourne for the past year – is one such Pesto groupie.

The 25-year-old has visited the penguin twice already and brags that she was a fan before he became an online sensation.

“We walked in and there was just this big, fluffy penguin… and we just kind of fell in love.

“He deserves to be famous.”

Olivia Wilson, who has been managing Pesto’s demanding media schedule for the aquarium, jokes that she’s not sure what’s the bigger drawcard to Melbourne this weekend: Pesto or the Australian Football League grand final – a sporting event taken so seriously that the entire state of Victoria gets a public holiday the day before to celebrate.

“You name a country, and he has had a mention in the media… there are very few places across the globe that haven’t had a bit of Pesto love.”

By her metrics, Pesto has reached an audience of about 5 billion and appears to have dethroned Thailand’s adorably erratic baby pygmy hippo as the internet’s favourite animal.

“People are trading him as Bitcoin, which has been unbelievable,” Ms Wilson says.

“Move over Moo Deng, basically.”